py3: work around the scope of exception variable in dispatch.run()

https://stackoverflow.com/questions/29268892/

mercurial/dispatch.py

@@ -79,12 +79,14 b' def run():'
     err = None
     try:
         status = (dispatch(req) or 0) & 255
-    except error.StdioError as err:
+    except error.StdioError as e:
+        err = e
         status = -1
     if util.safehasattr(req.ui, 'fout'):
         try:
             req.ui.fout.flush()
-        except IOError as err:
+        except IOError as e:
+            err = e
             status = -1
     if util.safehasattr(req.ui, 'ferr'):
         if err is not None and err.errno != errno.EPIPE: